1. Field of this Disclosure
This invention relates broadly to oilfield exploration. More particularly, this invention relates to apparatus and methods for expediting the downhole sampling of formation hydrocarbons via formation modification.
2. State of the Art
One technique utilized in exploring a subsurface formation is to obtain samples of formation fluid downhole. Tools such as the MDT and the CHDT (both trademarks of Schlumberger) tools are extremely useful in obtaining and analyzing such samples.
The MDT tool or other sampling tools typically include a fluid entry port or tubular probe cooperatively arranged within one or more wall-engaging packers for isolating the port or probe from the borehole fluids, one or more sample chambers which are coupled to the fluid entry by a flow line having one or more control valves arranged therein, means for controlling a pressure drop between the formation pressure and sample chamber pressure, and sensors for obtaining information relating to the fluids. Examples of sampling tools may be found in U.S. Pat. No. 3,104,712 to Whitten, U.S. Pat. No. 3,859,851 to Urbanosky, and U.S. Pat. No. 4,860,581 to Zimmerman et al., which are hereby incorporated by reference herein in their entireties). The sensors may include pressure transducers for monitoring fluid pressure and temperature. In addition, optical sensors may be supplied by an OFA, CFA or LFA (all trademarks of Schlumberger) module in order to determine the phase, the chemical composition, etc, of the fluid being admitted into the tool.
The use of the CHDT tool is similar in various aspects to the user of the MDT tool, but mostly in cased boreholes. The CHDT tool includes a mechanism for perforating the casing with a drilling mechanism (see, e.g., “Formation Testing and Sampling through Casing”, Oilfield Review, Spring 2002 which is hereby incorporated by reference herein in its entirety) and for plugging the casing after testing. The CHDT tool may alternatively be used in open hole, for example with modifications as shown in U.S. Patent Application Pub. No. 2005/0279499 or U.S. Patent Application Pub. No. 2006/0000606, both assigned to the same assignee of the present invention, and both included herein by reference.
The MDT and CHDT tools in their normal applications are used to obtain formation oil samples with a low viscosity; typically up to 30 cP. In certain circumstances, oils with a higher viscosity have been sampled, but the sampling process often requires several adaptations and can take many hours. It is believed that the maximum viscosity of an oil that has been sampled using an MDT or CHDT tool is approximately 3200 cP.
It will be appreciated by those skilled in the art that exploitation of more viscous hydrocarbons is becoming increasingly important due to the depletion of conventional low viscosity hydrocarbon reserves. Sampling viscous oils for reservoir characterization is very challenging as oils with a higher viscosity have a low mobility. Thus, depending on the local circumstances, viscous oils are very difficult to pump out of the formation. In fact, the low mobility of these oils often results in very long sampling times or makes it impossible to retrieve a representative sample, for example, because of the formation of emulsions. In some cases, the low mobility of these oils even makes it impossible to retrieve a sample. In addition, if sampling times are too long there is an increased probability that the tool will get stuck in the borehole.
Tools and techniques have been proposed for sampling heavy oils and bitumen, for example as shown in International Application Publication No. WO2007/048991, assigned to the same assignee as the present invention, and incorporated by reference herein.
While straddle packers mounted on the sampling tool above and below a sampling port, or large diameter packer can improve the flow of oil into the sampling tool, there is still a need for sampling tools and sampling methods that can be used, amongst other things, for sampling viscous hydrocarbons.